In recent years, with the advances in digital technologies employed for electronic apparatuses, the amount of data on music, images, information, and the like has increased. On this account, large-capacity and nonvolatile memory devices for storing such data are in increasing demand. As one of the measures to respond to this demand, a variable resistance nonvolatile memory device (referred to as the “ReRAM” hereafter) which includes a memory cell having a variable resistance element (nonvolatile memory element) that has a resistance value reversibly changing in response to a given electrical signal and keeps the resulting state even after power-off has received attention. Examples of the reasons for the attention include that the configuration of the nonvolatile memory element is relatively simple and thus implemented easily at a high density, and that consistency with a conventional semiconductor process can be easily ensured.
In general, a variable resistance element includes a variable resistance layer positioned between a pair of electrodes, and is classified roughly into two kinds according to a material used for the variable resistance layer (a variable resistance material). One kind is a variable resistance element which includes a perovskite material (such as Pr(1−x)CaxMnO3 (PCMO), La(1−x)SrxMnO3 (LSMO), or GdBaCoxOy (GBCO)) as the variable resistance material, as disclosed in Patent Literature 1 (PTL 1) for example.
The other kind is a variable resistance element which includes a binary transition metal oxide as the variable resistance material, as disclosed in Patent Literature 2 (PTL 2) for example. As compared with the aforementioned perovskite material, the composition and structure of the binary transition metal oxide are extremely simple. On account of this, the composition control and film formation at the time of manufacturing can be easily implemented. In addition, because of the advantage of relatively excellent consistency with the semiconductor manufacturing process, a great deal of research have been conducted in recent years.